This invention relates to rotary well drilling tools, and more particularly, to a hole opener or reamer with replaceable cutters. This application is related to U.S. Pat. application Ser. No. 593,036, entitled "Mounting for Inboard Cutters on a Raise Drill," filed July 3, 1975, by Robert L. Dixon and Malcolm D. Maxsted, and assigned to the same assignee as this application.
In earth boring reamers or hole openers, a series of rock boring cutters are rotatably mounted on a drive stem in such a manner that the outer edges of the cutters extend radially beyond the supporting structure so that the cutters enlarge or ream a hole of sufficient diameter to leave clearance for the upper portion of the hole opener. The rock boring cutter assemblies are preferably replaceable since these elements wear out and the drive stem can be reused.
These replaceable cutter assemblies conventionally have a fixed shaft on which a rock boring cutter is mounted for rotation. A variety of techniques have been used for mounting the shaft of the cutter assembly on the drive stem of the hole opener. In some embodiments the ends of the shaft are bolted or clamped in place. The principal technique utilizes a main pin which runs lengthwise of the mounting shaft on which the cutter is journaled for securing the shaft to the drive stem. Typically a diagonal bore and counterbore in the drive stem receive an end of the main pin and the shaft respectively. Retainer pins are used for securing the main pin and preventing its rotational and axial movement. Retainer pins permit minor movement of the pin and shaft with consequent fretting and wear.
The outer end of the shaft is supported by a mounting leg integral with, welded to, or otherwise secured to the drive stem. The outer end of this leg cannot extend beyond the outer edge of the conical rock boring cutter since clearance must be provided between it and the wall of the hole being reamed. Thus, the supporting leg is quite thin in the region of the bore for the outer end of the main pin. Since this results in small bearing surfaces, the bore in the leg is subject to excessive wear. Cracking and breakage through the main pin hole can result in premature failure of the drive stem. Flats or other means are provided on the leg for preventing rotation of the shaft. These regions eventually become deformed as the hole opener is used since they do not always mate tightly.
In the aforementioned co-pending application there is described a raise drill in which a cutter assembly is mounted by inserting the cutter journal shafts in a generally U-shaped opening between the mounting member and the drive stem. The inner end of the shaft is slidably fitted into a bore in the drive stem and the cutter assembly is locked in place by bolts and an inserted shim or spacer. The cutter mounting arrangement for a hole opener or reamer has close spacing requirements not present in the cutter mounting arrangement for a raise drill. It is therefore desirable to provide an easily mounted replaceable cutter assembly for a hole opener which does not subject the drive stem and mounting leg of the hole opener to excessive wear.